In the refining process to which the grinding discs according to the invention are particularly applicable, the pulp stock or grist is ground in a grinding space defined between a pair of discs which rotate relative to one another in an environment of fluid medium. Each disc comprises disc segments disposed annularly about the discs and is provided with ridges and grooves which shear the fibers of the grist in grinding-like fashion. The pulp material, which may consist of wood chips, bagasse, fiber pulp or similar fibrous material, is fed by a screw feeder or the like through an opening in the central portion of the stationary grinding disc into the "eye" of the grinding space and from which it is propelled by the centrifugal force generated by the rotational movement of the discs towards their periphery, where the grist is ejected with greatly accelerated force into the surrounding casing.
In order to generate the necessary centrifugal force to accelerate the stock from the inner central portion of the grinding space radially outwards and to obtain the desired degree of defibration and operating capacity in the grinding space, a high rotational speed must be imparted to the discs, such as on the order of 1500 r.p.m. to 3600 r.p.m. However, the resultant relatively high centrifugal force required to accelerate the stock from the inner disc portion, which determines the capacity of the apparatus, concommitantly subjects the grist as it progresses radially outwards to the outer disc portion to a progressively intensified centrifugal force. This intensified centrifugal force will accelerate the outward radial speed of the grist to such a degree that, unless special measures are taken to hold back the grist in the outer disc portion, the grist will be ejected prematurely from the grinding space, in only partly-treated condition, with consequent impairment of the defibration efficiency of the grinding apparatus. This problem becomes even more accentuated when steam or other vapor is generated during the grinding operation, as the result of high power input or dryness of the grist. The steam or other vapor will then flow with the grist outward through the grinding space between the discs and further accelerate the radial flow of the grist. As the centrifugal acceleration exerted on the grist is porportional to the disc diameter, as well as to the square of the r.p.m. of the disc, according to Newton's law of force and motion, the larger the diameter of the disc in the apparatus, the greater will be the problem of controlling the flow of the grist through the outer portion of the grinding space. Depending on application and capacity demand, grinding apparatuses used today normally have a disc diameter ranging between 20" and 64". Even if the larger diameter discs should be rotated at relatively slow speeds varying between 900 r.p.m. and 1800 r.p.m., they will still produce a centrifugal force of acceleration on the grist in the order of 700 g' s to 2800 g's. Assume, for example, that a disc rotating at 900 r.p.m. generates a centrifugal force of 700 g's; if the r.p.m. should be increased to 1800 r.p.m., the centrifugal force will be increased by a factor of 4, thus generating an increased centrifugal force of 2800 g's.
While discs of large diameter are desirable for capacity reasons, they require large amounts of energy, which is partly wasted because of their high peripheral velocity and consequent intensified centrifugal force, which renders the peripheral portion of the grinding space substantially ineffective for defibrating purposes. In addition, the high peripheral velocity of these large discs creates a serious noise problem.
Because of increasing demand for large capacity defibration equipment with adequate refining efficiency, it has proved to be a problem in the industry to properly control the radial passage of the stock between the outer part of the opposed grinding disc segments so as to obtain maximum performance. It should be understood that, as the stock progresses through the radial passage, it migrates alternately between the grinding segments on the opposing discs, and the more work on the stock in a single pass, i.e., the longer the dwell time in the grinding space, the more efficient and economical becomes the refining process. Unless the stock flow is properly retarded, the movement of the pulp becomes too rapid, as explained herein, and the defibrating action is minimized. Heretofore, attempts have been made to retard the passage of the grist through the grinding space by arranging the ridges and grooves in the grinding segments so that they can serve additionally as flow retarders. Such attempts are exemplified by applicant's U.S. Pat. Nos. 3,674,217, dated July 4, 1972, and 3,974,491, dated Aug. 17, 1976; and U.S. Pat. No. 3,040,997 granted to Donald A. Borden on June 26, 1962, U.S. Pat. No. 3,125,306 to E. Kollberg et al and U.S. Pat. No. 1,091,654 to Hamachek.
While these ridges and grooves serve to retard the flow, they still do not provide full utilization of the entire working area of the grinding space, since the grooves or channels between the ridges are spread out over a greater area at the periphery than at the inner portion of the grinding space. Furthermore, they do not solve the problem associated with high peripheral velocity of the presently-used large-diameter discs.
Another attempt to solve the problem of controlling the flow is exemplified by co-pending application Ser. No. 713,433, filed Aug. 11, 1976, in the name of Bo A. Ahrel, now patent No. 4,090,672. The primary object of that invention is to solve the problem created by the high pressure steam in the peripheral zone of the grinding space. In order to prevent the partly defiberized stock from being blown out from the peripheral grinding zone by the high velocity steam, Ahrel utilizes the centrifugal force to separate the steam and to open up an escape passage for the steam while retaining the steam-liberated stock between the opposing grinding surfaces.
Other examples of prior art are U.S. Pat. Nos. 1,098,325, 1,226,032, 3,684,200 and 3,845,909; and, German Patent No. 1,217,754 and Swedish Patent No. 187,564.